In recent years, nonaqueous electrolyte batteries such as lithium ion secondary batteries have been developed as a high energy density battery. These nonaqueous electrolyte batteries are expected as the electric sources of hybrid electric vehicles and electric vehicles. Also, these nonaqueous electrolyte batteries are expected as an uninterruptible power supply in a mobile base station. For this, these nonaqueous electrolyte batteries are desired to have other performance such as quick charge/discharge performance and long-term reliability. A nonaqueous electrolyte battery enabling quick charge/discharge has such a merit that charge time is very short and also, the engine performance of a hybrid vehicle can be improved and also, regenerative energy of the engine can be recovered efficiently.
The quick charge/discharge is made possible when electrons and lithium ions are quickly transferred between the positive electrode and the negative electrode. In a battery using a carbon type negative electrode, a dendrite of metal lithium sometimes precipitates on the electrode when quick charge/discharge is repeated. Dendrite causes internal short circuits with the result that there is a fear of heat generation and ignition.
In light of this, a battery has been developed which uses a metal complex oxide in place of a carbonaceous material. A battery using lithium-titanium oxide as a negative electrode active material enables stable quick charge/discharge and therefore has the performance that it has a longer life as compared with a battery using a carbon negative electrode.
However, the lithium-titanium oxide has a higher potential based on metal lithium (nobler) than the carbonaceous material. In addition, lithium-titanium oxide has a low capacity per weight. The battery using lithium-titanium oxide has a problem concerning a low energy density.
For example, the electrode potential of lithium-titanium oxide is about 1.5 V (vs. Li/Li+) which is higher (nobler) than the potential of a carbon type negative electrode. Because the potential of lithium-titanium oxide is caused by a redox reaction between Ti3+ and Ti4+ when lithium is electrochemically inserted or released, it is electrochemically limited. Also, because the electrode potential is as high as about 1.5 V, this allows stable operation of quick charge/discharge of lithium ions. It is therefore substantially difficult to drop the electrode potential with the intention of improving energy density.
With regard to the capacity per unit weight, on the other hand, the theoretical capacity of lithium-titanium oxide such as Li4Ti5O12 is about 175 mAh/g. On the other hand, the theoretical capacity of a general graphite type electrode material is 372 mAh/g. Accordingly, the volumetric density of lithium-titanium oxide is significantly lower than that of a carbon type negative electrode. This reason is that the number of sites absorbing lithium is small in the crystal structure of lithium titanium oxide and substantial capacity is dropped because lithium is easily stabilized in the structure.
In light of the above description, studies as to new electrode materials including Ti and Nb are being made. These materials are expected to have a high charge/discharge capacity. Particularly, a complex oxide represented by TiNb2O7 has a high theoretical capacity exceeding 300 mAh/g. However, it is necessary to calcine at a temperature as high as 1300 to 1400° C. to improve the crystallinity of a complex oxide such as TiNb2O7, giving rise to problems concerning low productivity and deterioration in quick charge/discharge performance.